1. Field of the Invention
The present invention relates to an image forming device for forming an image on a photo sensitive paper by scanning laser beams of three colors, red, green and blue, on the photo sensitive paper by a scanning device having mirrors, and by feeding the photo sensitive paper in a direction transverse to a scanning direction, and further by modulating a strength of the laser beams by synchronizing with those scannings. And, more particularly, the present invention relates to a mechanism for synchronizing a deflection scanning timing and a sending timing of an image data for an amplitude modulation.
2. Description of Related Art
Conventionally, in this kind of image forming device, a laser beam of three colors was scanned by a polygon mirror after the three beams became one beam, as described, for example, in A Continuous-Tone Laser Color Printer by Firth et al. (Journal of Imaging Technology, Volume 14, Number 3, June 1988). In this conventional image forming device, a photo sensor which reacts to a light having a specific wavelength was set to detect a part of the beam that was separated by a beam splitter from the scanning beam path and thus did not contribute to the image formation on the scanning lines. The timing of the sending of an image data was controlled based on a detection signal from the sensor. An automatic adjustment mechanism controlled by a computer was used so as not to cause the locational gap between the three colors of the laser beam.
However, it was costly and took time to adjust the laser beams to prevent a misconvergence on the formed image, because it is necessary to coincide the three laser beams very accurately to produce, in effect, one laser beam. Moreover, the automatic adjustment mechanism controlled by the computer was used so that the optics system would not be rendered inaccurate by factors such as an ambient temperature around the device. For the adjustment of the above optics system, an actuator of high accuracy was needed, so that the device became complex and expensive. Moreover, it was thought to use an invar alloy to reduce the expansion and contraction in the size of the parts of the device caused by temperature changes. However, the processing of invar alloy is very difficult so that the price of the device became very expensive as a result.